To make the sponge, add your sugar and butter together and cream until soft and almost white in colour. Then, add one egg and about a tablespoon and a half of flour and mix. Then add more flour as you go, this will stop the egg curdling. Next, add the next egg and the remaining flour and mix very well, making sure no lumps are present. Put this into a greased cake tin and put in the oven at around 180°C/gas 4. This will take roughly 20 minutes. But, depending on your oven, it might take a little longer. If you are unsure, take a sharp knife and poke the middle all the way through and pull it out. If there is liquid on the knife, it's not done. After your sponge has cooked, take it out and put it on a plate to cool. After this has cooled, you can make your butter cream. (Adding buttercream to a hot cake is NOT a good idea. ;) ) Take your butter and put it in a dish, then add your icing sugar a bit at a time, mashing it in with a fork. After all of it is added, it should be firmer than before and will look lighter in colour. Add your vanilla essence and mix it in. Cut your sponge in half. Spread one side with the jam, the other with your butter cream and serve with cream, ice cream, or custard. (Personally, I just like a sprinkling of lavender sugar. But, that's just me. ;) )[NOTE: If you find your mix too runny, add a little extra flour.]